As shown in FIGS. 3 and 4, a condenser microphone unit includes, as a general configuration, a unit case 10 and an electrostatic acousto-electric converter 20 housed in the unit case 10.
The unit case 10 is made of a metallic material such as aluminum or a brass alloy, and is formed in a cylindrical shape. On one end side of the unit case 10 directed to the sound source side at the time of sound pickup, a front acoustic terminal 11 is formed, and on the other end side thereof, an opening 12 for housing the acousto-electric converter 20 and the like is formed.
In this example, as shown in FIG. 3, the front acoustic terminal 11 is configured so that long first slit holes 11a and short second slitholes 11b, which are cut from the periphery on one end side of the unit case 10 toward the center thereof, are formed alternately. Aside from this configuration, in some cases, the front acoustic terminal 11 consists of one circular opening formed coaxially with the unit case 10.
The acousto-electric converter 20 includes a kind of condenser element configured so that a diaphragm 21 stretchedly provided on a support ring (diaphragm ring) 22 and a backplate 23 supported by an insulating seat 24 are disposed opposedly via an insulating spacer 25.
Since the condenser microphone unit of this conventional example is unidirectional, the backplate 23 consists of a porous plate. In the insulating seat 24, a sound hole 24a serving as a rear acoustic terminal is provided so that sound waves from the rear acoustic terminal 24a act on the back surface side of the diaphragm 21.
From the insulating seat 24, an electrode terminal rod 41 connected electrically to the backplate 23 by a conducting means (not shown) such as a lead wire is pulled out. The electrode terminal rod 41 is connected electrically to a gate electrode of a field effect transistor (FET) serving as an impedance converter (not shown) disposed in the opening 12 of the unit case 10.
In the unit case 10, a shielding member 30 that covers the front acoustic terminal 11 from the inner surface side is provided to prevent foreign matters and extraneous electromagnetic waves from intruding through the front acoustic terminal 11 (refer to Japanese Patent Application Publication No. 2004-343368).
Usually, as the shielding member 30, a metal mesh (a metallic mesh body) 31 is used. As the metal mesh 31 of this type, a stainless steel mesh (for example, wire diameter: 0.1 mm, #100 mesh, material: SUS304) has been used.
The metal mesh 31 is disposed on the inner surface side of the front acoustic terminal 11 and fixed thereto with an adhesive. To prevent the appearance from being impaired by the squeeze-out of adhesive, the metal mesh 31 is fixed to the inner surface of a front wall present in a central part of the front acoustic terminal 11 on one end side of the unit case 10.
The condenser microphone unit is assembled as described below. After the metal mesh 31 has been fixed as described above, the acousto-electric converter 20 is housed in the unit case 10, and a lock ring 27 having external threads is threadedly engaged with internal threads 13 formed in the inner wall of the unit case 10 and is tightened.
In the case where the unit case 10 is made of, for example, an aluminum material that is plastically deformed easily, the acousto-electric converter 20 is sometimes fixed by crimping the opening end edge in the rear part of the unit case 10 to the inside.
The shield due to the metal mesh 31 is effective against electric waves of VHF and UHF band used for broadcasting and the like. However, when a cellular phone, which has come into wide use rapidly in recent years, is used near a microphone, noise may be generated by the electromagnetic waves radiated from the cellular phone.
Specifically, the electromagnetic waves radiated from a cellular phone have high frequencies and short wave lengths, and in the range of several centimeters to several ten centimeters, the field intensity reaches several ten thousand times the field intensity produced in the city by commercial electric waves. Therefore, the electromagnetic waves radiated from a cellular phone reaches the backplate 23 through the openings in the metal mesh 31, whereby noise is generated.
One cause for this is that the metal mesh 31 is fixed to the unit case 10 exclusively with an adhesive, so that the electrical connection with the unit case is unreliable. Another cause for this is that the metal mesh 31 is plain-woven, and the electrical connection is made by intersections of longitudinal wires and transverse wires, so that the shield performance of the mesh itself is nonuniform.
Accordingly, an object of the present invention is to provide a condenser microphone unit in which the shield of a shielding member that covers a front acoustic terminal from the inside of a unit case is assured to prevent noise caused by electromagnetic waves radiated especially from a cellular phone from being generated.